This invention relates to feeding of strip material and more particularly to methods and apparatus for steering a travelling strip along a desired path.
There are many circumstances in which strip material must be fed along a linear path and in which it is desirable to provide some steering means whereby the strip material can be steered in a designed path without excessive wandering or skewing of the strip. In the steel industry, for example, there are instances in which steel strip must be fed forwardly, into processing equipment, often at high speed and in which a proper alignment of the strip must be maintained.
The present invention is particularly applicable to the feeding of metal strip produced from a continuous caster such as a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls.
After leaving the caster the hot strip may be passed to a coiler on which it is wound into a coil. Before proceeding to the coiler it may be subjected to inline treatment such as a controlled temperature reduction, reduction rolling, full heat treatment or a combination of such treatment steps. The coiler and any in-line treatment apparatus generally applies substantial tension to the strip which must be resisted. Moreover, it is necessary to accommodate differences between the casting speed of the twin roll caster and speed of subsequent in-line processing and coiling. Substantial differences in those speeds may develop particularly during initial start-up and until steady state casting speed is achieved. In order to meet these requirements it has been proposed to allow the hot strip leaving the caster to hang unhindered in a loop from which is passes through one or more sets of pinch rolls into a tensioned part of the line in which the strip may be subjected to further processing and coiling. The pinch rolls provide resistance to the tension generated by the down-line equipment and are also intended to feed the strip into the down-line equipment.
A twin roll strip casting line of this kind is disclosed in U.S. Pat. No. 5,503,217 assigned to Davy McKee (Sheffield) Limited. In this casting line the hot metal strip hangs unhindered in a loop before passing to a first set of pinch rolls which feed the strip though a temperature control zone. After passing through the temperature control zone the strip passes through further sets of pinch rolls before proceeding to a coiler. It may optionally be hot rolled by inclusion of a rolling mill between the subsequent sets of pinch rolls.
As noted in U.S. Pat. No. 5,503,217, strip passing from zero tension to a tension part of a processing line can wander from side to side. This is not acceptable and is overcome by the first set of pinch rolls being used to steer the metal strip into the tensioned part of the processing line. However, it has been found that standard pinch rolls are not properly effective to steer the strip and hold it against the tendency to wander. The pinch rolls can in fact contribute to misalignment and lateral movement of the strip if even small variations develop in the strip to roll contact pressure, the gap between the pinch rolls, or in the profile or cross-section of the cast strip passing between them.
Wandering of the strip not only results in misalignment of the strip in the down-line processing equipment, and it can lead to the transmission of twisting forces back into the hot strip issuing from the casting rolls. This twisting is particularly critical given the strip is at temperatures close to liquidus and thus the strip has little hot strength. In ferrous metal strip these temperatures are well in excess of 1100.degree. C. Thus such twisting can lead to hot lateral tearing of the strip just below the roll nip. In addition the generation of substantial fluctuations in the tensile forces at the edge margins of the strip leads to waviness in the strip margins and the generation of small edge cracks as the strip approaches the pinch rolls. In extreme cases it can even initiate severe lateral mechanical cracking and complete disruption of the strip. Accordingly, wandering of the strip in advance of the pinch rolls remains a critical problem, particularly in the casting of ferrous metal strip. The present invention provides a method and apparatus which can be applied to the steering of the strip in these circumstances to prevent excessive wandering and skewing of the strip. However, it will be appreciated from the ensuing description that the method and apparatus of the invention may be applied to the steering of strip material in other equipment and environments.